OpengL裁剪

在OpenGL中，裁剪用到glScissor:

void glScissor(
  GLint x,
  GLint y,
  GLsizei width,
  GLsizei height
);

Parameters
x, y
The lower-left corner of the scissor box. Initially (0,0).
width, height
The width and height of the scissor box. When an OpenGL context is first attached to a window, width and height are set to the dimensions of that window.
Remarks
The glScissor function defines a rectangle, called the scissor box, in window coordinates. The first two parameters, x and y, specify the lower-left corner of the box. The width and height parameters specify the width and height of the box.
The scissor test is enabled and disabled using glEnable and glDisable with argument GL_SCISSOR_TEST. While the scissor test is enabled, only pixels that lie within the scissor box can be modified by drawing commands. Window coordinates have integer values at the shared corners of framebuffer pixels, so glScissor(0,0,1,1) allows only the lower-left pixel in the window to be modified, and glScissor(0,0,0,0) disallows modification to all pixels in the window.
When the scissor test is disabled, it is as though the scissor box includes the entire window.
The following functions retrieve information related to glScissor:
glGet with argument GL_SCISSOR_BOX
glIsEnabled with argument GL_SCISSOR_TEST

例子：

void CShape3D::drawTriangle(void) {
    glClear(GL_COLOR_BUFFER_BIT);
    glLineWidth(2.0f);
    glBegin(GL_TRIANGLE_STRIP);
        glColor3f(0.0f, 1.0f, 0.0f);
        glClear(GL_COLOR_BUFFER_BIT);
        glVertex3fv(vTanglePoint[0]);
        glVertex3fv(vTanglePoint[1]);
        glVertex3fv(vTanglePoint[2]);
        glVertex3fv(vTanglePoint[3]);
    glEnd();

    glEnable(GL_SCISSOR_TEST);  
    glScissor(0,400 / 4, 400, 400 / 2);  
    glClear(GL_COLOR_BUFFER_BIT);  
    
    glFlush();
    glDisable(GL_SCISSOR_TEST);
    glutSwapBuffers();
}

cocos2dx的遮罩其实也可以利用这个来处理